Automatic volume control



A. F, VAN DYCK ET AL AUTOMATIC VOLUME CONTROL July 25, 1939.

led July 7,' 1950 lll? Patented July 25 1939 UNITED STiTlElSv PATENT OFFICE Goldsmith, New York, N. Y., assignors to Radio Corporation of America, a corporation of Delaware .Application July 7, 1930, `Serial No. 465,852

16 Claims. (Cl. Z50-20) The present invention relates to radio re- -ceiver reproduction control, and more particularly to improvements in automatic volume control of receivers.

It has become a common expedientto equip -a radio broadcast receiver with Van automatic gain, or volume, control arrangement in order to maintain the reproducer volume at approximately constant value with signal intensity levels which vary over a considerable range. While actual experience with such arrangements' reveals fair operation and effectiveness of automatic volume control, yet an operating disadvantage is often encountered.

When a receiver of this type is being tuned over its tuning frequency range, at all tuning )positions where no carrier is present the noise from static and other interference sources becomes highly undesirable. In fact, such increase in background noise level is effective in disturbing the nicety and comfort of operation of the receiver. This increase in background noise level is directly attributable to an inherent characteristic of the automatic volume control arrangement, since, in the absence of carrier, the automatic volume control increases the sensiytivity of the receiver to maximum, and therefore the response to background noises, and reproduction thereof, is at a maximum.

Now, We have discovered a method of, and devised means for, eliminating the increase in background noise level when a receiver, embodying an automatic volume control arrangement, is tuned through a point in its frequency range Where no carrier is present, the method basically involving rendering the signal translating means ineicient at such points.

Accordingly, it is one of the main objects of our present inventionY to provide a method of, and means for, operating a radio receiver provided with an automatic gain control so thatY the reproduction of undesirable noises is substantially eliminated during the tuning operation, the reproduction function of the receiver being rendered highly inefficient at such points' during tuning where the background noise level tends to increase.

Another important object of the invention is to provide an improved type of automatic vol ume control for receivers comprising means for impairing the operation of the receiver whenever the receiver is tuned through a point where no carrier is present, or where the carrier intensity becomes too weak to be useful, the said means including a device responsive to a predetermined decrease in carrier intensity to render the receiver audio arrangement inoperative or ineicient.

Another object of the invention is to provide in a receiver equipped with an automatic volume control, an arrangement including means responsive to a predetermined carrier intensity level decrease, and additional means, associated with the responsive means, for rendering the receiver reproducer inoperative or ineicient during said decrease.

Still another object of the invention is to provide in a radio receiver including tuning means, additional means, operative during the tuning process, to maintain the background noise level below a predetermined value.

And still another object of the invention is to provide in a radio receiver including a signal translating means, additional means for automatically rendering the translating means inef- @I particularity in the appended claims, the invention itself, however, as to both its organization and method of operation will best be understood by reference to the following description taken in connection with the drawing in which We have indicated one arrangement whereby our invention may be 'carried into effect.

Referring now to the drawing, wherein is shown diagrammatically a radio receiver of the superheterodyne type embodying an automatic gain control, the novel improvement is illustrated in oneof its many possible forms. The receiver circuit is conventional in nature, being well known to those skilled in the art, and comprises an antenna A grounded, as at G, the antenna being coupled, as at M1, to the tuned input circuit of the first radio frequency amplifier tube I. The latter is shown as a triode, but may be of the screen grid type, if desired.

The input circuit of the tube l is tuned, in the well known manner, by a variable condenser 2, the anode circuit of the tube being energized ,by the .source B. The-said anode circuit is coul art. u

tional manner, as shown.

The tuning condensers 2 in the two radlo stages and detector stage are uni-controlled, and are` shown thus by a dotted line. To secure the intermediate difference, or beat, frequency, oscila lations, from the local oscillator, are impressed on the input circuit of the detector tube Vlgthe local oscillations, of course, 'differing in frequency from the frequency to which the detector stage istuned, by the said Vintermediate fr requency, and a tuning condenser 2"- being of course provided in the local oscillator which is uni-controlled with condensers 2, as is well known in the AThe output circuit'of the tube 4 is coupled, as at M4, to the input circuit of tube 5, the input circuit being maintained fixedlytuned, by inductance 6 and capacity I, to the intermediate frequency. Thev circuit, Vincluding the'tube 5, acts as an .intermediate frequency amplifier, andit is, of course, understood that a plurality VYof such stages may be :employed in cascade.

After4 sufficient amplification of the intermediate frequency, the amplified yoscillations are impressed on the input" circuit of a second detector tube, through coupling' M5, the input circuit being tuned, by capacity 9, to the'intermediat frequency. 'The'tube rectifies the interme-4 diate oscillations by virtue of the well known grid leakldetection arrangement shown. Y

The audio'frequency pulsations, in the output circuit of the tube'a, are then amplified by an audio frequency amplifier.Y comprising tubes I0 and I I connected incascade', the'input circuit of tubel 0 being coupled tothe detector output circuit by an audio transformer Me. The output circuit of tube "I0 is coupled, by an audio transformer M7, to the inputcircuit of power tube II, and the output circuit of power tube II is coupled by transformer Ms to the reproducer (not shown). It is to be clearly understood thatV the receiving circuit just described is well known, and may readily' be substituted for in any of 'its component parts by any other 'well known parts of a receiver.' Furthermore, it should be noted Vthat the reproducer employed may be of any well knownY Again, the operation ofthe receiver is soA well known that from the diagrammatic showing, and the above description, the Y 'saine isi type.

obvious.

Now, in order'to maintaintheloutput volume of the receiver at a substantially constant level, an automatic gain lcontrol Varrangement is employed. 'I'his arrangement consistsV of an electron` dischar'getube YI5* having its control electrode connected by a lead I6, to the control electrode circuit of the second detector'tube 8, the lead including a Vcoupling capacity I'I. The anode. of the control tube I5 includes'. a stabilizing resistor I8, and is connected bya lead I9 to' a point 2| of Vpositive potential-(about 90 volts.)

on the power supply potentiometer 20. The

cathode'of Vthe control tube I5 isconnected,vby a lead 22,.to the low potential side of the second detector input circuit, and, by a lead 23, to a point 24 on the rpotentiometer2|), which point is The energizing current is derived from a'source 25, which is the volume controltube heater, the resistor 26 being tapped by a slidable member 21 and connected to the potentiometer 2Q, through a resistor 28, the latter being connected in series with the potentiometer resistance. The bias on the control electrode olf the control tube I5 can be adjusted by a contact 29, slidable on resistor 28, they contact being connected in series with a resistor 30.

The anodeof the control tube is connected,

I by a lead 3 I, to the low potential sides of the grid circuits of the radio frequency, and intermediate frequency, amplifier tubes. Blocking condensers 40 are inserted AVin these amplifier circuits between the cathodes and the tuned circuits. Thus, the anode current of the volume control tube determines the grid bias impressed upon the radio and intermediate amplifier tubes. Those. skilled in the art are well aware of the manner of maintaining the cathodes; of the controlled tubes at a relatively fixed voltage with respect to the control grids thereof. The present automatic volume control system, as described by G. L. Beers and.W.L.V Carlson in Ythe Proceedings of the Institute of Radio Engineers for March 1929,

page`5ll, would havevthe cathodes of the con-V to be' used with a radio receiver. AAgain, ,theV

grid potential of the Vcontrol tubeis adjustable by the slidable contact 29 on the resistance 28, and is also affected by the strength of the alternating current voltage fed 'to the control electrode ofthe control tube bythe signal from the Vinput circuit of the second detector tube 8.V K Y The Voperation of the automatic volume control arrangement should be obvious from the diagrammatic showingI in the drawing, it vbeing pointed out that the automatic gain control arrangement shown` herein( iswell known to-,those skilled in the art, and may be readily substituted for by any other type of automatic gain control device, such devices, essentially, controlling the sensitivityof the radio` or intermediate frequencyV amplifiers, or' both, in an inverse manner relative to the change in intensity level of the signal cartions iny control tube I5,Y and this in turn deter-V mines thea'mplifier grid bias and, amplification.

`It will, thus, be seen that if the signalV is removed, the amplification is increased, or in otherV Words thereceiver becomes more sensitive, and background noiseswhich may exist are easily reproduced. I Y

It is fundamentally necessary to have the volume control arrangement such that the less the signal intensityjthe greater the sensitiveness of the receiver, this being necessary in order to provide the desirable characteristics of automatic volume control, However, as clearly explained heretofore, it is the purpose of Ythe present invention to provide including fading prevention.V

an arrangement whereby when no signal at all is present, or one too weak to be useful, the sensitivity of the receiver shall be reduced, or, at least, that the reproduction of background noise shall be reduced or eliminated, by rendering the signal translating device ofthe receiver inefficient, or perhaps inoperative. 4Byrthe term signal translating arrangement, is meant not only the reproducer device, but also any ofthe element or elements subsequent in the circuit to arrangements whose function isfmerely vto amplify high frequency oscillations. Thus, the detector circuit, the audio amplifier circuit and the immediate reproducer circuit would be encompassed within the expression signal translating device.

The particular arrangement shown in the drawing for rendering the signal translating arrangement ineflicient whenever the signal carrier intensity decreases below a predetermined intensity level, depends upon the fact that the value of anode current of all the radio frequency and intermediate frequencyv tubes depends upon the signal carrier strength. For example, in the type o-f superheterodyne receiver shown in. the drawing, this current is from twomilliamperes to milliamperes. Thus, in this particular receiver, when the anode current reaches some value between `10and 15 milliamperes, the signal is no longer of practical usefulness. Therefore, anoverload relay, set to operate at a predetermined value, for example 13 milliamperes., is provided, the relay being arranged to render the reproducer inoperative whenever the carrier intensity level becomes too low to be-of any value. Again, the reproducer becomes operative only when there is sufficient signal strength to reproduce satisfactorily, without the rreproduction of background noises.

Specifically, these results are accomplished by connecting a mechanical relay device between the output circuit of the intermediate frequency amplifier, preferably the last intermediate stage, and the reproducer. The relay consists of a solenoid electromagnet 40 connected in series with the anode circuit of tube 5, a condenser 43 being connected across the solenoid 4B to `by-pass high frequency currents, and having a value suited to the frequency employed.

The direct current owing through the electromagnetis, therefore, the anode direct current of tube VA5, and the arrangement is so shown for the sake of simplicity of diagram. The circuit could be arranged, as ,would ordinarily be done in practice, so that the electromagnet would receive'the combined anode currentsof all of the tubes whose amplification is Vcontrolled by the automatic volume control means. In the diagram, this would be the total anode currents of tubes vi, 3 and 5. Use of the combined currents would permit use of a relay operable on larger currents, and therefore a less expensive or more reliable one.

The armature 44 of the electromagnet il is pivoted at .45, and `connected by a lead G6 to one side i of the secondary of the output transformer Ms.

The element 41, which is adapted to contact with the armature to short-circuit the secondary of the output transformer when the armature 4G is attracted towards the electromagnet 4B, is connected by a lead 48 to the other side of the power transformer secondary, through a resistance 49.

A stop 50 is disposed in alignment with the contact member 4l, the armature head 5i being disposed between the two. `A capacity 52 may be connected across the output transformer-secondary, andrgiven a value suitable to the particular transformer and reproducer used, to eliminate sparking at the contacts ell, 5l, this sparking being readily picked up by the receiver as interference clicks when the contacts open or close, unless guarded against in the manner shown. The resistance 49 has a value depending upon the particular transformer and reproducer used, and the degree of inefliciency of translating device desired. If the magnitude of this resistance is made zero, the sound reproducer will be short circuited when the relay contacts are closed, and will therefore be absolutely silent when no carrier is being received.

The armature de, of the electromagnet 4B, is normally tensioned by a spring Gil, means being provided to vary the tension of the spring, (such means being-conventionally shown by an arrow adjacent the fixed end of the spring) it thereby being possible to adjust the relay to operate at the desired current. It is pointed out that the specific arrangement shown herein is not in any way limiting, and that the relay device may be connected between the reproducer and the plate circuit of any of the radio frequency amplification stages or intermediate frequency amplifica.- tion stages.

The relay device de, t!! should preferably be a time-relay, so that it will actuate only as the ,l

current change which is desired to actuate it, has persisted for a certain length of time. In this way unnecessary short-circuiting of the reproducer is prevented, as for example during the process of tuning in a desired signal. tion of the background noise suppressor employed herein will be obvious from the arrangement as shownvin the drawing, and from the following specific explanation.

In the radio and intermediate frequency tubes, the values of anode current flowing depend upon the grid ,voltage values. The grid voltage values are determined ,by the automatic volume control tube or other automatic volum-e control means, in line with the normal operation of usual automatic volume control arrangements. The auto-matic volume control tube has its effect determined by the strength of the signal carrier fed to it.

Therefore, there is a direct relation between signal carrier strength and the value of anode current flowing in the tube or tubes controlled by the automatic volume control means. Usually this relation is that the stronger the carrier the small-er the anode current. In the diagram shown, tube 5 might have, for example, a current of three milliamperes when no signal was being received. This current would flow through the electromagnet @il of the relay. When a signal carrier is tuned in, its voltage appearing in the input circuit of tube 8 would also be applied t'o the grid of tube l5, whose anode current flowing through resistance I8 would be changed thereby.

The change in anode current through resistance 20, produces a change in the potential applied to the grids of the amplifier tubes through lead 3l. The change in potential of these grids changes the amplifying ability of these tubes, and also the values of anode current in them. Therefore, the relations are such that the signal carrier automatically adjusts the amplifying ability of the amplifier to deliver a substantially constant radio frequency voltage output to the detector tube 8. In so doing however it causes changes in the anode direct currents. In the The opera- I,

example shown, Where the anode current of `tube "5 maybe 3 milliamperes when no signal ris being received, the anode current may, for example, be reduced to 2.0 milliamperes when a signal carrier ,of moderate strength is received, and 2,5 milliamperes when a very Weak carrier is received.

Assuming that considerable interference is present, the signal accompanying the 2.5 milliamperes current may be useless. Therefore, complete operation of' the receiver is desirable when this current is less than 2.5 milliamperes and no operation is desired when this current exceeds 2.5 milliamperes. If We employ a relay so adjusted that itrwill close and make contact between elements 4.1 and 5l When the current exceeds 2.5 milliamperes, and opens When the current is less than 2.5 milliamperes, the desired Yoperation will be achieved, and the receiver will be operative in the presence of signals stronger than the predetermined desiredV value, and inoperative on weaker signals or no signals. Interference noises function as no signals, in effect on the relay,rbecause'of their intermittent nature,

Vand so are eliminated as the receiver is rendered inoperative when they only are incoming. Means may be provided for adjusting the relay operation vpoint (2.5 in theV above example) to any value suited to particular reception conditions.

The salient points in the operation of noise suppression are:

(a) When the receiver is tuned to a signal carrier, noise suppression is not desired or needed, and the suppression Yarrangement must be such as to leave incoming signals unaffected.

(b) When the receiver is not tuned to a signal carrier, only interference noises pass through the receiver and are reproduced as unpleasant sounds, unless suppressed. It is desired to suppress these sounds. Y.

(c) Current conditions in certain parts of the receiver circuit are different when signal carrier is present from conditions when signal carrier is absent. Thereforethese changes in current can be utilized to bring a noiseY suppression means into operation.

(d) 'Ihese current changes can effect noise suppressionrby being arranged to render thek receiver inoperative or inefficient at some point in its circuit beyond vthat part wherein the current changes themselves exist. Y

(e) In the particular arrangement previously described in detaiLthe currentrchanges .operate a relay Whose contacts short circuit the reproducer, and the arrangement is, of course, not limiting in scope.

Wherever the use of a solenoid relay is not prac-V ticable, it is to be understood that other means may be employed as a substitute therefor, or in conjunction therewith. For example, an electron discharge relay may be employed in conjunction with the mechanical relay the anode current change of Vthe electronic relay being utilized to actuate the solenoid relay as shown in the drawing. ,Howeven a solenoid relay will be found practicablewhenever the anode current issufficiently different under the two conditions of signal on and off, and has enough magnitude to actuate a solenoid relay of practicable design. Again, instead of' short-circuiting the output transformer secondarly'or the voice coil of the reproducer, the relay can be readily'employed for opening or closing the reproducer circuit, or for controlling means for rendering any of the signal translating elements inefficient in reproduction.

While We have indicated and described one arrangement* for carrying our invention into effect, it Willbe apparent to one skill-ed in the art that our invention is by no means limited to the particular organization shown and described, but that many `modifications may be employed without departing` from the scope ofV our invention asset forth in the 'appended claims.

What we claim is:

1. A signaling system` comprising a high frer- ,Y

quency amplifier circuit, means for tuning said circuit to a desired frequency, a gain control device forregulating the sensitivity of said amplifier, means for utilizing the amplifier output, and additional means connected directly in theY amplifier circuit for automatically' decreasing the utilizing efficiency of Ysaid last means When` said amplifier is adjusted to maximum sensitivity by said gain control device but less thanea predetei'L mined amount of high frequency lenergyY is received. Y Y

2. A signaling system comprising a high frequency amplifier circuit, means for tuning said circuit to a desiredfreque'ncy, a gain control device for regulating the sensitivity of said amplifier, means for changing the frequencies amplified Yat high frequency to a W frequency, a 10W frequency amplifier, and additional meansV directly responsive to current flow in said high frequency amplifier'circuit lfor automatically impairing theV said gain control device but no high frequency Y energy is received.

3. In combination in a radio receiver, means for tuning the receiver, a signalf translating'means,

and additional means, including an velectrical relay responsive to a predetermined decrease" in carrier intensity, for short-circuiting the signal translating means during tuning of the receiver through points on the frequency scale Where said decrease occurs. f

4. A signaling system comprising a high frequency amplifier circuit, means for tuning said circuit through a range of frequencies, a gain control device for regulating the sensitivity oi said amplifier, means for changing the frequencies amplified at high frequency to a low frequency, a 10W frequency amplifier, and additional means responsive to said regulated amplifier for automatically impairing the transmission efficiency of the lowV frequency amplifier when the high frequency amplifier is adjusted to substantially maximum sensitivity by said gain control device but is tuned through a point of the range Where no high frequency energy is received. Y

5. Areceiving system'comprising a signal translating device, a radio frequency amplifier connected to the device, means for regulating the sensitivity of said amplifier inversely with received signal energy level, and means, operative to disable said device, when said amplifier is adjusted to substantially maximum sensitivity by said regulating means and no signal energy is received said disabling means being connected directly in the amplifier circuit and beingresponsive to its sensitivity regulation.

6. In combination, in a superheterodyne radio receiver, a radio frequency amplifier, a frequency changing means, a low frequency amplifier, means for controlling the gain of the radio frequency amplifier, and an electrical relay, responsive to VVariations in the output of said frequency changing means, for reducing the transmission efciency of the low frequency amplifier When the efficiency of the low frequency amplifier when the gain of the radio frequency amplifier is increased and no carrier wave is received, said relay being connected between the frequency changing means and the output of said low frequency amplifier.

8. In combination, in a radio receiver, a radio frequency amplifier, a frequency changing means, a low frequency amplifier, means for controlling the gain of the radio frequency amplifier, and an electrical relay, responsive to variations in the output of said frequency changing means, for reducing the transmission efficiency of the low frequency amplifier when the gain of the radio frequency amplifier is increased and no carrier wave is receiver, said low frequency amplier including an output transformer, and Said relay being connected to short-circuit the transformer secondary.

9. A superheterodyne receiver comprising a radio frequency amplifier, means for tuning the amplifier to a desired frequency in a range of frequencies, a frequency changing device, an intermediate frequency amplifier, a second frequency changing device, an audio frequency amplifier, means for connecting said last amplifier to a reproducer, an automatic gain control circuit connected between the second frequency changing device and said radio frequency amplifier, and an electrical relay, connected between the output of said intermediate amplifier and said audio frequency amplifier, for reducing the transmission efficiency of the audio frequency amplifier when the radio frequency amplifier is adjusted for increased gain but is tuned through a point where no carrier wave is being received.

10. In a receiver for modulated carrier waves, a signal frequency amplifier, means for producing audio frequency voltage by rectification of amplified signals, means for producing direct current voltage by rectification of amplified signals, means for amplifying the said audio frequency voltage, means actuated by said direct current voltage to control the gain of said signal amplifier, and means, in said signal amplifier circuit directly responsive to signal amplifier current for suppressing the amplification of said audio frequency Voltage, for impressed signal voltages below a predetermined value.

11. In a system for the transmission of modulated signals, the combination with an amplifier for amplifying said signals, and means automatically controlling the gain of said amplified in accordance with the strength of received signals,

12. In a radio receiver of the type including a radio frequency amplifier having a tuned input network, an electron discharge tube demodulator and an audio frequency utilization network, automatic gain regulation means, responsive to variations in amplitude of signals impressed on the demodulator tube input, for decreasing the gain of said amplifier as the said amplitude increases, and audio transmission efficiency control means including an electrical relay, said relay being directly responsive to said variations and being connected to said audio utilization network in such a manner that the transmission efciency of the utilization network is substantially impaired when said amplifier gain is substantially a maximum.

13. In a signal receiving system of the type comprising means responsive to change in the strength of a received signal for automatic volume control, tuning and demodulating means, background noise suppressing means and means whereby the background suppressing means is controlled from the automatic volume control means.

14. The invention set forth in claim 13, characterized in that further means are provided for so adjusting the background noise suppressing means that it may be rendered insensitive to signals of less than a predetermined amplitude.

l5. In a system for the reception of energy, a receiver having a detector normally in operative condition, a demodulated signal output path associated therewith and normally maintained in a condition of disability, means adapted to regulate the gain of said receiver, and means for removing the disability from said demodulated signal output path, said last-named two means having a common element controlled by the carrier energy transmitted to said receiver and means whereby the second means is controlled from the gain regulation means.

16. In a carrier frequency signaling system tunable over a range of carrier frequency channels, an arrangement for suppressing interchannel disturbances and weak signals, comprising means for rendering said system inoperative when the signal strength is below cut-off level, signal responsive means for disabling said first means thereby rendering said system operative when the signal strength rises above said cut-off level, and adjustable means for selecting said cut-off level.

ARTHUR F. VAN DYCK. ALFRED N. GOLDSMITI-I. 

